This invention relates to pay television systems. It is the object of such television systems to encode the signal at the transmitter in such a manner that a receiver cannot furnish a picture unless a decoder is activated by the subscriber. Activation of the decoder of course leads to charges for the program received. In known methods and arrangements of the above-described types, the transmitted signal is encoded by varying the timing between the video and synchronizing components, that is selectively retarding or advancing the video component relative to the synchronizing signals. Key signals are then transmitted which indicate the necessary retardation or advance of the signal which must be effected in the receiver in order that the final system furnished to a paying subscriber may have the video portion of the signal in the correct relationship relative to the synchronizing portion.
In other known systems of the above-described type, the coding operates on the synchronizing portions of the signal. For example, the field synchronizing components of the television signal may be frequency modulated on the picture carrier, while the line synchronizing components are coded and then transmitted to subscriber receivers concurrently with the sound-signal components on a sound carrier. Key signals indicating the coding schedule of the line synchronizing components are transmitted to subscriber receivers over a separate channel. Both of the above-described systems have definite drawbacks. The first lends itself rather readily to unauthorized decoding, the second requires a great deal of extra equipment since a standard television transmitter cannot be used.